This invention relates to a process and an apparatus for devolatilizing polymer melts and more particularly the apparatus of the present invention is a polymer melt distributor for use within a reduced pressure devolatilizer.
In the manufacture of certain polymers, such as impact and crystal polystyrene, residual material including monomers, solvents or diluents such as ethyl benzene, and low molecular weight oligomers such as dimers and trimers may be present in the polymer coming out of the reactor. It is important to remove at least a major proportion of these residual materials from the polymer. Generally the polymer melt is passed through a preheater at high pressures and temperatures into a devolatilization chamber maintained at a high temperature and a low pressure. The residual materials then tend to diffuse through the polymer and escape to the surrounding atmosphere in the chamber, resulting in a polymer with a lower level of such residual materials.
The degree of removal of the residual materials depends on a combination of one or more of temperature, pressure, diffusion rate of residual materials through the melt, surface area of the melt, the extent of bubble formation within the melt and residence time of the melt in the devolatilization chamber.
In the prior art one approach to maximize the surface area of the polymer melt is to flow the polymer melt vertically down the surface of a plate heat exchanger as disclosed in U.S. Pat. No. 4,153,501 issued May 8, 1979.
A second approach is to use a series of falling strand devolatilizers as disclosed in U.S. Pat. No. 4,294,652 issued Oct. 13, 1981. A series or sequential devolatilizer permits the temperatures to be adjusted to increase devolatilization of the residual materials. While series devolatilizers are useful, they represent additional capital, operating and maintenance costs over single stage devolatilizers.
It is believed that one of the limiting factor in falling strand devolatilizers is the dwell time of the polymer in the form of a strand (e.g. the time it takes for a polymer to descend down a strand). Increasing temperature would increase the diffusion rate and bubble formation, however, the dwell time of the polymer in the strand would decrease due to lower polymer viscosity. Thus, there is a point of diminishing return in heating the melt, not to mention the problem of polymer degradation.
What is needed is a method to increase bubble formation and to hold the polymer melt in a devolatilizer for a slightly longer period of time in a condition such that a large surface area of polymer is available for devolatilization of the residual materials. Preferably the polymer melt would be in the form of a foam to increase the surface area of the melt.